The content of opsin was lower in rod outer segment preparations from miniature poodles carrying the gene for an inherited progressive retinal atrophy than it was in normal dogs. This deficiency was found in very young poodles even before electroretinogram or ultrastructural differences could be observed. Opsin synthesis in the affected dogs appeared normal, however. These determinations were made by gel electrophoresis of outer segment proteins following intravitreal injection of radioactive leucine.